A device for measuring a breathing gas volume flow is now present in practically all respirators (also known as ventilators). The so-called hot wire anemometry has proved to be an especially sensitive method for measuring the volume flow. A thin, so-called hot wire, whose resistance depends on the temperature, is arranged in the flow path of the gas of the hot wire anemometry measuring device. The hot wire is cooled by the flow depending on the intensity of the flow, so that the resistance of the wire at a defined current flowing through the wire is an indicator of the volume flow of the gas with which the gas is flowing past the wire. Hot wire anemometers are therefore preferably used because they themselves cause only a slight pressure loss within the flow channel. However, they have the drawback that the signal, which contains the information on the volume flow, is very weak.
A Wheatstone bridge circuit is frequently used for the analog evaluation of such a weak electric measured signal. The measuring element in the form of the hot wire is an element of the bridge circuit, but this measuring element is often not located in the immediate vicinity of the other elements of the bridge circuit. A cable connection is therefore necessary in these cases. The cable connection must, however, have a high-quality design in order to keep the resistance associated therewith as low as possible and as reproducible as possible.
Besides the problems concerning the cable connection, the coupling of the cables with the sensor (hot wire) itself is a source of additional possibilities of error, since the output signal may be directly affected by very low additional resistances, which may be caused, e.g., by welding and soldering resistances as well as cable and plug resistances. On the other hand, it is difficult to reach high reproducibility during welding or soldering operations. Thus, precisely these sources of additional resistance do represent a great problem when the change in the resistance of the hot wire is to be determined with precision.
In addition, the measured signal of a hot wire anemometer is affected not only by the volume flow but also by the absolute temperature of the gas flowing past and the composition of that gas.
It is known in this connection that the resistance of the hot wire and that of a temperature compensation wire are evaluated with a common bridge circuit during the measurement of the breathing gas volume flow. Both the hot wire and a second, unheated wire (temperature compensation wire), whose resistance is an indicator of the absolute temperature of the gas, are part of the bridge circuit in such a device.
If the direction of the gas flow is also to be determined besides the absolute value of the volume flow, it is necessary to also evaluate the signal of a second hot wire, in which case this second hot wire is arranged, unlike the first hot wire, in the shadow of a flow resistance such that a greatly reduced volume flow is admitted to the second hot wire when the flow takes place in a first direction, whereas this reduction does not take place in case of the opposite direction of flow. As a result, the direction of flow can be inferred from the measured cooling of the second hot wire compared to the first hot wire.
Thus, especially if the direction of flow of the gas is also to be measured, there will be a large number of connections between the measuring electronic unit, on the one hand, and sensors arranged in the flow channel, on the other hand, which are all subject to the above-mentioned problems of the undefined contact resistances, so that the entire measurement of the volume flow and of the direction of flow of the gas contains considerable sources of error. This problem is not limited to the use of hot wire anemometers, but it also occurs in the case of other electrically sensitive sensors for volume flow measurement in a flow channel.